Microwave radiometric aircraft landing assist system

ABSTRACT

A microwave radiometric landing assist system is provided which serves as an aid for landing aircraft, for example, and which may complement the present practice of using visible light sources for runway guidance. The landing assist system of the present invention involves the use of passive metallic objects positioned on the ground in a predetermined pattern. These objects are detected by a microwave radiometric receiving system located in the aircraft, and which likewise may be considered passive in that it does not radiate energy from the aircraft. The microwave radiometric landing assist system of the invention provides a display for the pilot which is analogous to the display provided by visible light sources, for example, but which is available day and night and under all types of weather conditions. The information presented to the pilot is obtained at a rapid rate over an area which includes the metallic objects pattern. The area frame rate is fast enough to allow the pilot to react properly in flying the terminal approach.

United States Patent 1 Caruso, Jr. 1 Apr. 3, 1973 [54] MICROWAVERADIOMETRIC AIRCRAFT LANDING ASSIST SYSTEM g [57] ABSTRACT [76] Inventr! Philip J- s J 15611 Del A microwave radiometric landing assist systemis pro- Prado Drive, Ha ienda Heights, vided which serves as an aid forlanding aircraft, for

Calif- 91745 example, and which may complement the present [22] Filed:May 15 1969 practice of using visible light sources for runway guidance.The landing assist system of the present inl l PP 824,860 ventioninvolves the use of passive metallic objects positioned on the ground ina predetermined pattern.

52 us. Cl. ..343/100 ME, 343/108 R These Objects are detected by amicrowave radiomet- 51 no.0. ..G0lw 1/00 ric receiving system located inthe aircraft. and which [58] Field of Search ..343/100 ME, 108 Rlikewise y be considered passive in that itdoes not radiate energy fromthe aircraft. The microwave [56] References Cited radiometric landingassist system of the invention provides a display for the pilot which isanalogous to the UNITED STATES PATENTS display provided by visible lightsources, for example,

2,502,974 4 1950 McElhannon... ..343/l12RX but which is available y andnight and under all 2,458,654 i/i949 Southworth ..343/l00 ME UX types ofweather conditions. The information 2,944,151 7/1960 Whitney et al..343/l08 R X presented to the pilot is obtained at a rapid rate over3,067,283 an area which includes the metallic objects pattern.

12/1962 Petritz et a]. ..250/83.3 HP X Primary ExaminerSamuel FeinbergAssistant Examiner-Richard E. Berger Attorney-Jessup & Beecher The areaframe rate is fast enough to allow the pilot to react properly in flyingthe terminal approach.

5 Claims, 12 Drawing Figures PATENTEUAPM ma SHEET U UF 7 Emma mmmmm 1.2, M 3 m 5 W184. i c

PATENTEUAPRS 197a SHEET 5 OF 7 04/4 ,etaraer I Mar/)1 f Arm fiecef/er Asis well known, the control of air traffic around an I airport isacomplex-problem which is becoming more so in ..recent years. A majorfactor involved is the guidance-of theaircraft into a successful landingas the runway is approached,.and just before the aircraft actuallycontacts the'runway. 'Since traffic is a continual problem, it is mostimportant that every aircraft be brought inquickly,.safely-andefficiently, and without delays .due to-adverseweather conditions.

. @Improvements in *modern electronic guidance con trolsystems permitaircraft to approach runways at more-rapid rates,and.at more closelyspaced intervals, --even at night and under adverse weather conditions.

However, the occasion often arises that even though the aircraft couldbe brought into a landing pattern under a particularset of weatherconditions, the visibility is such that final'touchdown is perilous, sothat landings are not attempted. This gives rise to con- ;gestion,delays andresulting hazards. The need exists, therefore, for an aid toassist in landing aircraft during times of low visibility, forexample,when visibility is ;less than 100 feet.

The system ofthepresent invention is intended to assist present dayelectronic instrument landing systems and to be used in conjunctiontherewith, to provide the pilot with an effective visible display of aground pattern, as'he. actually approaches the runway. As mentionedabove, such a display has been achieved in the past by providing lightsources in a predetermined pattern along the runway. However, theselight sources are often obscured under various weather conditions. Inthe practice of the present invention, in one of its aspects,'the lightsources are complemented by passive metallic objects.

One advantage in-the use of passive metal objects on the ground is-theirutter simplicity, and the fact that they require little ornomaintenance. The metal objects are detected by a microwave radiometricsystem in the aircraft, which also is of a passive nature since it isnon-radiating. The airborne radiometric system pro- LII for thereflectors. Theairborne system likewise is passive, inthat it isnon-radiating so as to alleviate any radio frequency interferenceproblems with other equipment in the aircraft. The displayinformationmay be readilypresented to the-pilot in the cockpitin a realistic'mannerso as to simulate actual visual observation of the runway or runwayextension. Moreover,'the system of the invention is economical'andrelatively in- Y expensive, so that it has general utility.

The landing assist system of the invention isprecise in its operation,in that it can establish ;glide slope, azimuth approach and yaw, all to"within 1 degree. Moreover, the system is capable of providing asecondary check'on altitude of the aircraft during landing to withinapproximately ten percent. Although thesystem is to be described hereinas a landing assist for aircraft,

, it may also be used as a'potential-aid to'prevent collivides a displaywhich analogizes the actual visible light scanning type, and it may beof the multiple beam variety so as to increase information rate andsensitivity. The landing assist system of the invention provides imagesof the metallic objects on the ground in a visual display, and directlyprovides the pilot with guide slope,

: yaw, and azimuth approachinformation, all on an instantaneous basis.

Briefly stated, the landing assist system of the invention exhibits apassive requirement on the ground in the form of simple reflectors, withno need for personnel, air or maintenance. Also, the system exhibits aminimum requirement for additional, if any, land area sions, to providean artificial horizon, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a black bodyradiation curve;

FIG. 2 is'a schematic representation of radiometric temperature sources;

FIG. 3 is a block diagram of a typical prior art microwave radiometer;

FIG. 4 is a sketch illustrating the boundaries within which energy maybe received in the airborne microwave radiometric receiving system ofthe invention from a metal ground object;

FIG. 5 is a schematic representation of a normal glide path of a landingaircraft, and the elevation "projection of the scanning pattern of themicrowave radiometric receiving system mounted in the aircraft;

FIG. 6 is a schematic representation of the azimuth scan of the airbornemicrowave radiometric syste'r'h'as the aircraft moves along the glidepath of FIG. 5;

FIG. 7 is a view of a runway, having metallic objects placed along itsapproach section in one possible 'pattern in accordance with oneembodiment of the invention;

FIG. 8 is a schematic representation of successive displays which appearon the displayfdevice in the airborne microwave radiometric receivingsystem, as the aircraft approaches the runway of FIG. '7 for a landing;

FIG. 9 is an orthographic representation, partly in DETAILED DESCRIPTIONOF THE ILLUSTRATED EMBODIMENT Radiometric techniques have been usedfor'many years, particularly at the infrared wavelengths for'detectionsystems. The system of the present invention, however, is predicatedupon the use of the microwave wavelengths which extend, for example,from30 GI-Iz to 300 MHz, at which the techniques differ from those used inthe infrared region.

slightly depending upon the percentage and types of objects within thebeam.

From black body theory, any perfectly absorbing body emits radiations atall frequencies according to Plancks radiation law. A curve of relativeradiated energy versus wavelength as derived from Plancks radiation lawis shown in FIG. 1. It can be seen that within the infrared region ofthe spectrum, the energy emitted is relatively high and is proportionalto the fourth power of its ambient temperature T. At microwavefrequencies, on the other hand, the energy varies only directly with T.The energy is much less at the microwave wavelengths than at theinfrared wavelengths, as shown by the curve of FIG. 1, but the amount ofenergy is still significant and the difference in energy levels betweenthe infrared and the microwave wavelengths is overcome by increasedmicrowave radiometer sensitivity as compared with the infraredradiometer.

A major difference between infrared and microwave radiometric techniquesis that the material parameters play a much more significant role atmicrowave wavelengths than the actual temperature. The apparenttemperature being radiated to and collected by the antenna (FIGURE 2) ofthe microwave radiometer is a combination of apparent temperatures fromthree sources. The apparent temperature T may be represented by thefollowing equation:

T=eT,,+pT,+'rT (1) where: e p and 1- are the emissivity, reflection andtransmissivity coefficients respectively; and T T, and T are the ambienttemperature-of the material, the reflected sky temperature and thebackground temperature respectively. For most metallic materials r T,,is zero and is ignored.

Of the several types of prior art microwave radiometers, the mostcommonly used at present is the Dicke radiometer which is shown insimplified block form in FIG. 3. The Dicke radiometer compares theenergy at the antenna 12 with that from a reference load 14, byalternately switching between the antenna and the reference load bymeans of a switch 16. Both the antenna and reference signals areconverted to a selected intermediate frequency in a mixer state 18, byheterodyning them with an appropriate signal from a local oscillatorsource 20. The resulting intermediate frequency signals are amplified inan intermediate frequency amplifier 22, of one or'more stages, anddetected.

Other types of radiometric receivers are also appropriate. Theseinclude, for example, cooled or uncooled parametric amplifiers, tunneldiode amplifiers, and'the like. The Dicke radiometer appears to be themost economical.

The resulting video signals are then amplified in a video amplifier 24,of one or more stages. The video amplifier is followed by an appropriatesynchronous detector 26 to assure thatonly the desired signals areutilized, the synchronous detector being sensitive only at the switchingrate of the antenna-reference load switch 16. The resulting output fromthe synchronous detector 26 is integrated in a usual integrator 28 toprovide the output data.

Even though the time sharing between the antenna and the reference load14 reduces the signal by 50 percent, the Dicke type of radiometer anddetection process enhances the detection of weak signals and reducesunwanted noise and gain fluctuations.

The microwave radiometer signal differs from that ordinarily encounteredin radar or communications systems in that it is both random in phaseand amplitude. That is, the microwave radiometer is required to detect anoise-like signal in a noise background. However, it is importanttoobserve that successive scanning of multiple targets in a systematicpattern, such as proposed for the landing system of the presentinvention provides percent pattern probability of detection, andprevents any possibility of false pattern occurrences.

Since it may be desirable to obtain pitch or yaw information in thesystem of the invention with greater accuracy than the beam resolution,beam splitting techniques may be used to increase accuracy, as will bedescribed.

The diagram of FIG. 4 assumes that an aircraft equipped with a microwaveradiometric receiver, in the practice of the present invention, ismomentarily located at a point A over the ground designated B, and itsradiometer antenna is a split beam multiple beam type and is scanning apattern directed to the ground B located within the boundaries D" and E.A metallic reflector object 100 located on the ground B is tiltedtowards the point A, as shown. Energy from the sky, designated T, isreflected from the tilted metallic reflector 100 within the boundariesD" and E, as shown by the vector T Also, energy emitted by the groundwithin the boundaries "D and E" is designated by the vector T If A 9represents the total areas intercepted by the scanning radiometricantenna, and A, represents the area of the reflector 100,v then theparameter A,/A a 1 percent is adequate for positive detection of themetallic object 100. However, in order toassure complete reflectoridentification at all times, a ratio of AJA 9 10 percent appears to beappropriate.

In FIG. 5, a normal glide path of the aircraft of 5, for example, isdenoted. In the particular illustration, it is assumed that a microwaveradiometric receiving system is included in the aircraft, and theantenna beam is split into, for example, 20 beams of 2 each, so as tocover a fan in elevationof, for example, 40 between a pitch angle of 25and 65. The fan in the particular example shown in FIG. 5 is illustratedat altitudes of 500 feet, 400 feet, 300 feet, 200 feet and 100 feet.However, it will be understood that the actual scan is a continuoussweep of the ground as the aircraft flies along the flight path of FIG.5 and comes in to a touch down.

As shown in FIG. 6, for example, the fan of the split antenna beams isalso swept in azimuth, for example, i 20 about the center line of theglide path, so as to describe the illustrated patterns as the aircraftflies along its flight path. Again, although the patterns for 500, 400,300, 200 and 100 feet are shown in FIG. 2B,

it will be understood that the radiometric system in the aircraftactually makes a continuous sweep of the ground as the aircraft comes infor a landing.

In the practice of the present invention, in one of its embodiments, andas shown in FIG. 7, a plurality of the metal reflectors 100 referred toin FIG. 1 are laid out along the approach to a runway in adjacent rows.In the ly two feet to twenty feet in diameter when circular discs areused; or from 2 feet to feet, for example, along each side when squarediscs are used. In a typical installation, 2% foot circular discs areused with a twenty foot separation, at the 200 foot altitude position.

As will be described, the microwave radiometer located in the aircraftincludes a display device located in front of the pilot to provide him areadily observable display as the aircraft is brought in for a landing.As the aircraft approaches the runway of FIG. 7, the displays observedby the pilot are of the form shown in FIG. 8. For example, the firstdisplay A seen by the pilot when the aircraft, for example, is at the500 foot altitude along the flight path of FIG. 5, consists of two rowsof the metallic objects 100, as described in conjunction with FIG. 7.

Likewise, in the position B, the two rows of the reflectors 100 arestill being scanned. Then at the position C which, for example, maycorrespond to the 300 foot level in the glide path of FIG. 5, the threerows of reflectors 100 are visible; whereas at the 200 foot level, asshown in the view D", all five rows of the reflectors 100 are visible onthe display. Finally, at the 100 foot to touchdown levels of theaircraft, as shown in the display D of FIG. 8, the five rows of themetallic objects 100 remain visible.

It will be appreciated, of course, that the pattern of FIG. 7 of thereflectors 100, and the corresponding displays of FIG. 8 are merelyexemplary of one particular pattern which is appropriate, and wherebythe pilot is provided with a reliable'and simple means of determiningthe altitude and attitude of his aircraft as he approaches the runway.Any discrepancy in yaw or azimuth is readily apparent from the displayof FIG. 8, and the number of reflectors appearing in the displayprovides a measure of his altitude. The displays of FIG. 8 may appear,for example, on a 5 inch screen of a displaydevice.

Although five separate displays are shown in FIG. 7, as representativeof the actual displays when the aircraft, for example, is at 500, 400,300, 200 and 100 feet, it will be appreciated that the actual displayobserved by the pilot will be continually changing from instant toinstant as the pilot flies along the flight path of FIG. 5. It should benoted, for example, that as the aircraft's altitude changes from 500feet to 400 feet, the series of lines of reflectors 100 will separate asa function of time, and this separation may be used as a check onaltitude.

It should also be noted that with the particular pattern of FIG. 7, whenthe height is 300 feet, for example, as shown by the display C, thereare three lines of reflectors on the display. This, of course, iscompletely arbitrary. It may be used as a means to indicate to the pilothis altitude at any given time. The third line of reflectors, of course,does not appear suddenly at 300 feet, but slides into the displaystarting from the top of the screen at 400 feet (B) and moving down thescreen as the aircraft flies between the 400 feet (B) and 300 feet (C)altitude. The additional lines of reflectors which appear on the screenat'the 200'feet (D) level also slide into the display in the samemanner.

It should be reiterated and emphasized that the particular patternsselected, as shown in FIG. 7, and the particular displays and scanningprocesses utilized in the illustrated embodiment of the system hereinare merely illustrative, and other appropriate patterns and scanningprocedures maybe selected. As an example, the detected metallic objectsmight be displayed as points rather than small areas or broadened linesas shown.

An airborne microwave radiometer for use in the practice of theinvention is shown in FIG. 9. The system illustrated is a ten beamradiometer, and the antenna 200 is a lens antenna which acts as its ownradome. The system is housed in an appropriate housing 202, and

ten radiometer receivers 204 are mounted as shown,

each with its probe or feed 206 being located at the focal point of theantenna. The receivers are mounted on a yoke 210 which is pivotallymoved back and forth by an appropriate drive mechanism (not shown) toachieve the scanning function described above.

The major parameters of the system of the invention, with reference'to aparticular embodiment, may beas follows. These parameters, of course,are given merely by way of example, and are not intended to limit theinvention in any way. For example, the number of beams used in thesystem, the scan width, the observation an-' gle, and the type ofdisplay may be varied to suit the particular requirements of anyspecific installation.

Type of System Multi-Beam Scanner Number of Beams 10 Field of View 14 X40 Frame Rate 10 per second Type of Display Intensity Modulated CathodeTube Operating Frequency Range Ka Band System A T (1.0 secondintegration time 0.25K

Approximate Weight 45 pounds Approximate Size 20 inches X 20 inches X 35inches I Power Requirements 115 volts, 2 amperes A block diagram of thearea imaging microwave sensor which is suitable for use in the landingsystem of the invention is shown in FIG. 10. As mentioned above, thesystem in its illustrated embodiment comprises a ten beam microwavescanner, a data interface, and a simple display. The multibeam lensantenna 200 produces ten independent beams displaced over i 5 beamwidthsfrom the lens axis. The wide scanning angle desired for.

may include a polyrod radiator mounted at the end of a circular waveguide. Ten such probes are mounted with their respective receivers 204on the yoke 210, as shown in FIG. 9. The assembly is then rotated backand forth as described above, by the mechanical drive designated 300 inFIG. 10, up to 1:20 in the transverse plane. The probes 206 form an arcwith its center at the focal point of the lens, and all the probes pointat the focal point. When scanned in the transverse plane, the path isalso an arc with its center at the focal point of the lens.

Each feed assembly 206 for each microwave radiometer receiver consistsof a polyrod radiator, mounted in the end of a circular wave guide. Thepolyrod is approximately two inches long in a constructed embodiment,with the phase center occurring near the middle of the rod. This feedassembly, when integrated with two ferrite rotators, constitutes theentire microwave portion of the radiometer channel. Ten such feeds andferrite rotators are mounted with their respective receivers on the yokeshown in FIG. 9. The rotators, for example, may be i 45 Farradayrotation units which require i milliamps through their coils forswitching. The two rotators in each feed are separated by a polarizingload. The assembly then functions as a usual horizontal or verticalpolarization selector, controlled by a Dicke switch. The entire rotatorassembly in each feed is approximately four inches long. Thepolarization selector is used to allow the scanning mechanism to operateeither in the horizontal direction or the vertical direction, whilemaintaining sensitivity to vertical polarization in both conditions.

The received microwave signal energy is amplified in each receiver 204,which is a sensitive microwave radiometer receiver. In the presentinstance, for example, and as shown in FIG. 10, each radiometer receiver204 is coupled to a demodulator 302, and the demodulated signal isapplied through a data interface 304 to the display 306 in the aircraft,and also, if so desired, to a data recorder 308. The data interface 304also generates an automatic gain control signal for the receiver, alsoshown in FIG. 10.

The individual radiometer receiver 204 is shown in more detail in FIG.11. The receiver of FIG. 11 is a broad band superheterodyne receiverwith a square law detector, video amplifier, and automatic gain controlcircuit. The energy from the antenna 206 is passed through a Dickeswitch 400 to a broad band wave guide balanced mixer 402. A localoscillator 404 supplies its output to the mixer 402 through apotentiometer 406, and the resulting intermediate frequency signal isamplified in an intermediate frequency amplifier 408. The localoscillator 404 may be a Gunn effect, semiconductor type oscillator, andthe intermediate frequency amplifier may be a low noise broad bandtransistorized circuit.

The output of the intermediate frequency amplifier 408 is applied to thehigh sensitivity square law detector 409. The detector is followed by athree stage low noise transistorized video pre-amplifier 410 whichincorporates an AGC circuit 412. The AGC circuit may consist of a shuntconnected field effect transistor attenuator. A video post amplifier 414is provided consisting of cascaded operational amplifiers to completethe video amplifier section. The portion of the receiving systemdescribed above is mounted on the yoke 2110, for example, as representedby the blocks 204 in FIG. 9.

The demodulator portion 302 of the radiometer channel contains twoidentical units. These units each consist of a synchronous detector 420followed by an integrator 422. By proper switching of the synchronousdetectors it is possible to derive a wide variety of combinations of theinput signals. The choice of two demodulator sections in the illustratedembodiment is used to obtain two independent data inputs Ty and T forthe two antenna input ports.

The display 306 suitable for use in the landing system of the inventionproduces a simple image of the area scanned. The image is oriented withrespect to the antenna boresight. Other types of images can be generatedwith the same basic equipment by the addition of appropriate scanningfunction generators. The resulting distorted images primarily would beuseful in enhancing the visual effects to a pilot, either by causing theimage to be oriented with respect to the ground, or by over-emphasizingperspective effects, etc.

The display system is shown in block form in FIG. 12. It includes aclock generator 500 which generates all the timing signals used in thedisplay system. The clock generator has two operational modes, afree-running mode and a synchronized mode. The synchronized mode permitsthe injection of an external clock signal into the system to synchronizethe system with others, such as the data recorder 308 of FIG. 10, orother data recording system. The basic clock signal from the clockgenerator 500 is counted by a decade counter to obtain the binary codeddecimal signals necessary to identify the ten radiometer channels beinginterrogated in the particular system illustrated herein. The binarycoded decimal signals are produced by a computer position binary counter502 and are introduced through appropriate computator drive stagesrepresented by the block 504 to a computator 506. Then, for successiveclock pulses from the clock generator 500, the commutator positioncounter block 502 causes the commutator drive 504 to switch thecommutator 506 from one radiometer channel to the next.

It will be appreciated that the binary coded decimal type of code isused in the particular display of FIG. 12 because of the fact that inthe illustrated embodiment, ten simultaneous input lines are handled.Other types of coding would be used for other numbers of radiometers inthe system.

By the control of the commutator position block 502, the ten channels ofradiometer input are sequentially multiplexed and supplied through anamplifier 508 to the Z axis drive amplifier, as designated by the block510. The Z axis drive amplifier 510 amplifies the signals from thecommutator, corrects for non-linearities in the cathode-ray tubephosphors, and inserts the beam intensity information into the Z-axiscoupling of the cathode-ray tube in the variable persistenceoscilloscope display represented by the block 512.

Since the commutator position is determined by the output of the digitalcounter in the block 502, a simple digital-to-analog converter 514 isused to generate the vertical position signal for the cathode-ray tubein the display 512. This signal is applied through a summing network 516and through a vertical drive amplifier 518 to the cathode-ray tube inthe display. The vertical scan signal rapidly scans the beam across theface of the cathode-ray tube, following the commutator across theradiometer input signals. These inputs are arranged so that the datafrom the radiometers will appear in the proper place on the display.

A usual direct-current position potentiometer (not shown) is included inthe antenna scanning mechanism to generate voltages proportional to theantenna position. This potentiometer provides the position of the centerof the two sets of antenna beams. Correction to this average positionmust be made for the staggering of the beam positions. By properlyarranging the inputs to the commutator 506, the least significant bit ofthe commutator position counter 502 carries the information necessary toobtain this correction.

That is, for example, a 1" in this bit indicates the left feed positionand a indicates the right feed position. This bit is used to add orsubtract a small adjustable voltage by way of the block 520 and througha summing network 522 to the horizontal position signal derived from theaforesaid potentiometer, and which is applied to the cathode-ray tube inthe display 512 through a drive amplifier 524.

The normal spot size on the cathode-ray tube in the display 512 is muchsmaller than the antenna beam width size. The spot size may becontrolled independently by potentiometers 526 and 528 interposedbetween an oscillator 530 and the adders 516 and 522. The oscillator 530may be a simple 1 Mhz sawtooth oscillator, whose output is added to boththe horizontal and vertical sweep signals, in an amount determined bythe settings of the potentiometers 526 and 528.

The invention provides, therefore, an improved microwave radiometric lowvisibility landing aid system for aircraft, and which is particularlyadvantageous in that the ground elements may be of the passive typerequiring a minimum of maintenance, and in that the radiometric systemin the aircraft does not involve the radiation of energy which wouldhave a tendency to interfere with other equipment in the aircraft. Also,the system is advantageous in that it is essentially simple andinexpensive. As described, patterns of the ground objects may be used,so as to provide an indication of where e p and 1- are the emissivity,reflection and transmissivity coefficients respectively, and T,,, T, and

T are respectively the ambient temperature of the material, thereflected sky temperature and the background temperature; radio-metricmicrowave receiving means mounted in said vehicle for detectingradiations over an area including the generally horizontal faces of saidmetallic objects formin said (passive radiation emissive means, saidradiations eing lrectly proportional to the aforesaid apparenttemperature (T) of the radiation emissive means; and display meanscoupled to said microwave radiometric receiving means for providing adisplay of said passive radiation emissive means to the pilot of thevehicle.

2. The system defined in claim 1 in which said particular groundlocation constitutes a runway extension for landing the airbornevehicle, and in which said metallic objects are placed in adjacent rows.

3. The guidance system defined in claim 1 in which said radiometricreceiving means includes a lens type antenna, and at least oneradiometric receiver positioned at the focal point of said lens antenna.

4. The guidance system defined in claim 3 and which includes drive meansmechanically coupled to said receiver for reciprocally moving saidreceiver angularly through a predetermined range effectively scanningsaid receiver over an area and through ,a predetermined antenna pattern.

5. The guidance system defined in claim 3 and which includes a pluralityof radiometric microwave receivers mounted at the focalpointof said lensantenna.

1. A guidance system for an airborne vehicle including: passiveradiation emissive means placed in a particular ground location, saidpassive radiation emissive means including a multiplicity of separatemetallic objects placed in a predetermined pattern on a particularground location each having a generally horizontal and generally flatradiation face positioned to assume an apparent temperature (T) inaccordance with the equation T Epsilon Ta + Rho Ts + Tau Tfb, whereEpsilon , Rho and Tau are the emissivity, reflection and transmissivitycoefficients respectively, and Ta, Ts and Tb are respectively theambient temperature of the material, the reflected sky temperature andthe background temperature; radiometric microwave receiving meansmounted in said vehicle for detecting radiations over an area includingthe generally horizontal faces of said metallic objects forming saidpassive radiation emissive means, said radiations being directlyproportional to the aforesaid apparent temperature (T) of the radiationemissive means; and display means coupled to said microwave radiometricreceiving means for providing a display of said passive radiationemissive means to the pilot of the vehicle.
 2. The system defined inclaim 1 in which said particular ground location constitutes a runwayextension for landing the airborne vehicle, and in which said metallicobjects are placed in adjacent rows.
 3. The guidance system defined inclaim 1 in which said radiometric receiving means includes a lens typeantenna, and at least one radiometric receiver positioned at the focalpoint of said lens antenna.
 4. The guidance system defined in claim 3and which includes drive means mechanically coupled to said receiver forreciprocally moving said receiver angularly through a predeterminedrange effectively scanning said receiver over an area and through apredetermined antenna pattern.
 5. The guidance system defined in claim 3and which includes a plurality of radiometric microwave receiversmounted at the focal point of said lens antenna.